2 x 1080Ti vs 1 x 2080Ti. So much difference?
The reason why his viewport rendered preview mode is so much closer to the final result is that BlenderGuru has set his Viewport Sampling to be much higher. Higher samples means less noise and lighting artifacts. Since he essentially has twice the computing power, he can afford to crank up the samples in the viewport much higher without dealing with too much lag.

You could probably also increase yours since you still have a 2080ti. In the second screenshot on the right-hand side, under Sampling, increase the value next to Viewport. Turn it up as high as you like, just take care not to crank it up so high such that the preview becomes too laggy.